1. Field of the Invention
The present invention relates to an image forming method wherein an image is formed by electrochemically insolubilizing an image forming material by application of electric current using an aqueous coloring material dispersion, particularly an aqueous fine particle coloring material dispersion, and an image forming material used in this method.
2. Description of the Related Art
Many methods of using a liquid image forming material are known in image recording technologies used in offices. For example, silver salt recording technology, ink jet recording technology, and electrophotographic recording technology.
Printing technology are known using silver salt has been published in Tanemura Hatsumi et al., "HIGH QUALITY COLOR COPYING SYSTEM BY SILVER SALT PHOTOGRAPHIC METHOD", Preprint of Japan Hardcopy '89, p229. Printing technology using liquid development electrophotographic technology has been published in E. B. Caruthers et al., "Modeling of Liquid Toner Electrical Characteristics", Proceedings of IS & T 10th Int'l. Congress on Advances in Non-Impact Printing Technologies, p204('94). Printing technology using ink jet recording has been published in many technical reports such as Usui Minoru, "Development of a New Process MACH", Preprint of Japan Hardcopy '96, p161.
In other conventional printing technologies, the recording method using silver salt does not present problems of image quality or image fastness. However, the use of this method in an office is associated with problems, because this method is based on a printing process which involves a chemical reaction using chemically active agents and producing waste. The problems of ink jet printing technology are that high resolution cannot be easily obtained because of problems created by the small nozzle diameter in printing reliability; and that image fastness, safety, and printability on plain paper are inadequate, because the image forming material is usually an aqueous dye. Electrophotographic technology has no problem with regard to image quality, printability on plain paper, and high image fastness comparable to that of a printed product. However, electrophotographic technology has the disadvantages that a large amount of energy is consumed in the fixing unit of an electrophotographic apparatus; that the size of the apparatus is large because of the complicated printing process; and that safety and reliability problems arise.
There are also disclosed as conventional technologies resembling the image forming method of the present invention, for example, a technology using an electrodepositable liquid in which a coloring material is dispersed in an electrically insulating liquid so as to generate an electrical double layer in Japanese Patent Application Laid-Open (JP-A) No. 7-181750 and Japanese Patent Application Publication (JP-B) No. 7-54407; "Fine Pattern Forming Method" relating to an electrodeposition printing technology using a printing plate comprising an electrically conductive substrate with an insulating pattern formed thereon in JP-A No. 4-9902; and "Electrodeposition Offset PrintingMethodand Printing Plate" in JP-A No. 6-293125, respectively.
Meanwhile, properties required for printing technology for use in offices are, for example, a color high-quality image of 600 DPI or higher and multi-gradation, capability of printing on plain paper, high image fastness comparable to that of a printed product, high safety of the recorded matter and the recording apparatus, almost no waste produced, and low running cost. However, conventional technologies have various defects as described above, and none of the technologies described in the above-described various patents documents using electrodeposition property have reached the level at which the above-mentioned requirements are completely satisfied.
In order to obtain an image having a high quality (resolution of a level of 1000 DPI, good color reproduction, and multi-gradation), the thickness of the image structure is preferably 2 microns or less, and more preferably 1 micron or less, in the light of the relationship between the range of color reproduction and the sharpness of image. Accordingly, the average particle size of the image forming material, as the element which imparts structure to the image, needs to be of a sub-micron order. However, since a fluidity problem arises if the average particle size of the image forming material is 5 .mu.m or less, practical use of a powdery image forming material is difficult. In comparison, when viewed from this standpoint, the use of a liquid image forming material would be effective. In the step of forming an image having a size of the order of several microns, it is technically difficult to accurately control the image formation of the particles of image forming material if the size of the particles is very small. Accordingly, use of aqueous solution of a dye having a size of molecular order which is a minimum fine particle as an electrodeposition material is considered to be a very effective technical method also from the viewpoint of an accurate coloring material controlling method.
The electrophotographic technology, which uses a developer in the form of an electrically insulating liquid and which is disclosed in, for example, JP-A No. 7-181750, is advantageous in that high resolution can be obtained because the size of the image forming material is of a sub-micron order; and printability on plain paper is high. However, since this method uses a developer comprising a hydrocarbon solvent, the solvent vapor presents a serious safety problem. Therefore, the use of such a developer is strictly regulated in some countries.
The electrodeposition printing technology using a printing plate comprising an electrically conductive substrate and an electrically insulating pattern formed thereon as described in patents such as JP-A No. 4-9902 and the like is disadvantageous in that it is difficult to change image patterns for each printing because the process is complicated, for example, the non-image portion of an insulating resist needs to be prepared beforehand by photolithography. Other disadvantages are as follows. Since the apparatus to be used in this method is provided with a high precision, has a large size, involves many steps and produces a large amount of waste, it necessarily follows that the place where the apparatus is installed for printing is limited to a factory well equipped with facilities. Further, since the hysteresis of the image forming step tends to remain on the substrate, minute image recording cannot be fully reproduced. Furthermore, since the image forming portions in this method are concave, it is difficult to obtain a high quality image, because the selective adherence of particles to the image portions by electrophoresis is weakened and a large proportion of the liquid component of the image forming liquid material tends to remain on the image forming portions thus reducing the viscosity of the image forming material in the image forming portions, and consequently the image forming material in the image forming portions is liable to flow or cause a cohesive failure in the transfer step. Further, any of them is a technology for making a printing plate used for forming fine patterns, and is not a technology of recording an image directly on a recording medium.
As described above, technologies of conventional image forming methods cannot satisfy the properties, i.e., a high degree of safety using a simple apparatus, required for printing for use in offices.
In order to realize a high quality (600 DPI or more and multi-gradation), the minimum unit size of image forming material is preferably 1 micron or less. Therefore, it is necessary to use a liquid image forming material which can contain fine particles of a coloring material. When the installation of an apparatus in an office is taken into consideration, the liquid for use in the image forming material needs to be extremely safe. Preferably, water should be selected as the liquid as there are no problems regarding its safety. A printing process using a printing plate which cannot be regenerated is not suitable for use in an office, because office printing technology is required to prepare various sorts of printed matters in small quantities, easily, and at a low cost. Therefore, the market has a liking for a system which comprises inputting an image signal to produce image information for every printing and the image information as an image of the image forming material is transferred for recording on various recording media such as plain paper and the like corresponding to various need of uses.
In addition, when such factors as the fastness and high optical density of an image, and the safety from the fact that the color forming material is not taken into human body are taken into consideration, it is necessary to use a coloring material comprising a pigment. It is also necessary to keep the energy consumption to a minimum in the printing step.
Further, a pH regulator is often added to an aqueous dispersion for reducing energy required for deposition of an electrodeposition material. It has been known that if this pH regulator remains in the deposited electrodeposition film, various problems occur such as re-dissolution of an image (electrodeposition film) and the like from the viewpoint of image stability.
The present invention has been made to address the aforementioned properties. Therefore it is an object of the present invention to obtain a high-quality image by use of an electrodeposition material containing a pigment coloring material, particularly fine particles of a coloring material and to provide an image forming method which is able to form an image even by application of voltage wherein potential difference between image formation part and a reference electrode generated corresponding to photoimage signals, is extremely safe and simple and has a wide area of use as well as to provide an image forming material which is suited for the method.